QUESTION: My 1975 Suzuki GT550 exhaust rotted out, and I, being the tinkering type, wanted to try something different. The motorcycle gets about 46 mpg in town and 38 mpg on the highway. My question: Can I replace the exhaust system with something that will “come on the pipe” at highway rpm (and perhaps increase the efficiency at this speed) instead of 6,500 rpm where peak horsepower is at? I ran the numbers to determine the length, but I would like to know your thoughts before I fabricated something like this.
ANSWER: If I were concerned about the fuel consumption of a 39-year-old machine, one of the first things I’d do is replace the needle jet. At part-throttle, this is what controls your fuel flow, and having had the needle rattling back and forth inside it at every suction pulse for all those years, it has surely become somewhat enlarged. After that, I would look for a partially blocked air filter element, and perform a basic tune-up.
Do your carbs have black plastic floats or soldered brass ones? Shake the brass ones next to your ear to see if they have taken on any liquid. Black floats were manufactured back when gasoline did not contain the hefty dose of aromatic components (toluene, xylene, etc.) that it has had since the late 1970s. The black plastic float in my 1976 Dodge van soaked up aromatics in 1978 to such an extent that it sank, causing stumbling idle and heavy fuel consumption. Set float levels, set ignition timing, and be sure spark plugs are of the right heat range and gapped correctly.
Does one cylinder smoke at idle? If so, it might have a failed seal and be drawing in gearbox oil (TZ-750s did this on the center two cylinders when the crank bolts there got loose).
What you propose to do—build a tuned pipe whose peak is at highway cruising operation—would be just the ticket if you were planning to build an exhaust system for a constant-speed generator engine. Can you ride your machine like that? Motorcycles are supposed to be fun!